1. Field of the Invention
The invention relates to a stabilizer composition which is suitable for stabilizing an olefinically unsaturated monomer during production, purification and storage, and to a corresponding process for stabilizing an olefinically unsaturated monomer.
2. Discussion of the Background
During the preparation of olefinically unsaturated monomers, for example ethene, butadiene, isoprene, vinyl acetate, (meth)acrylic acid, (meth)acrylates, acrolein, acrylonitrile or vinyl-substituted aromatics, these olefinically unsaturated monomers are subjected to several purification process steps, for example distillation or extraction, in order to remove undesired by-products or impurities. The production and distillation process steps in particular are performed at elevated temperatures.
Olefinically unsaturated monomers therefore have a tendency to unwanted polymerization as early as during the preparation and/or purification process. The risk of polymerization exists in all abovementioned monomers—particularly at elevated temperature. Some of these olefinically unsaturated monomers, for example butadiene, however, even during storage or in the course of transport, also have a tendency to a spontaneous, usually strongly exothermic and therefore hazardous polymerization.
However, the comparatively creeping polymerization of olefinically unsaturated monomers during production and purification is also undesired. Firstly, it results in deposits of the polymers in the reactors and columns, and secondly in a reduction in the amount of available monomers. Deposits of the polymer can lead, among other results, to reduced heat transfer in individual plant parts, and hence to a reduced productivity.
In addition, plant components, for example filters, can become covered and be blocked with the undesired polymer. This has the consequence of unplanned interruptions of production, in order to be able to carry out cleaning of the plant. Every shutdown firstly causes repair and cleaning costs; secondly, a shutdown also causes a production shortfall, and so it is always attempted to avoid them or to minimize their number as far as possible.
Consequently, additives, which are referred to either as polymerization inhibitors or as retarders, are added to the olefinically unsaturated monomers generally as early as during the preparation process. Polymerization inhibitors are, as the name actually states, capable of completely preventing undesired polymerization. Polymerization inhibitors are, however, consumed rapidly, and so the polymer content rises just as significantly within a short time as if no additive had been added. Polymerization retarders, in contrast, can never completely prevent polymerization, but rather only slow it. At the same time, they are consumed significantly more slowly than polymerization inhibitors.
The presence of both types of polymerization inhibition in monomer production is justified. Constant supply of fresh polymerization inhibitors can achieve the effect that the polymerization content is kept at a very low level or polymerization can be prevented completely during a production process proceeding without disruption. Polymerization retarders are, in contrast, of great importance in the case of stoppage of the additive supply, since, as a result of their longer activity, they still prevent significant polymerization even when the polymerization inhibitors have long since been consumed. In general, both types of additives are used in combination with one another.
Polymerization inhibitors, which are frequently described in the literature are, for example, so-called stable free nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) or derivatives thereof. The polymerization retarders used are generally nitroaromatics, for example 2,4-dinitro-6-sec-butylphenol (DNBP), 2,4-dinitrophenol (DNP) or 4,6-dinitro-ortho-cresol (DNOC). Nitroaromatics exhibit good retarder properties, but also possess serious disadvantages. For instance, they are generally highly toxic and possess carcinogenic, mutagenic and/or reproduction-toxic properties. The use of these nitroaromatics therefore entails correspondingly high safety precautions on the part of the user. Furthermore, in the event of incineration of the nitroaromatic-containing residues of the distillation columns, environmentally harmful NO gases are released. Attempts are also made to avoid this as far as possible.
To prevent polymerization in the preparation of vinylically unsaturated compounds, as well as the abovementioned substance classes, there are also many further additives which are known from the literature and can be used, for example C- and/or N-nitroso compounds, hydroxylamines and oximes. All of these substance classes, just like the nitroaromatics, have quite a high proportion of undesired nitrogen atoms which can leave as NOx in the later incineration process of the distillation residues.
A further known substance class for preventing this undesired polymerization is that of quinone methides of the formula I:

The use of this substance class for inhibiting the polymerization of styrene is described by Bacha et al. in U.S. Pat. No. 4,003,800 and also in U.S. Pat. No. 4,040,911, where the substituents of the R′″ and R″″ type may be hydrogen, an alkyl group, a cycloalkyl group or an optionally alkyl-substituted phenyl group.
EP 0 737 659 and EP 0 737 660 also describe quinone methides for stabilization of monomers, the quinone methides used in EP 0 737 659 having hydrogen as the substituent of the R′″ type, and aryl or heteroaryl groups which may optionally have further substituents as the substituent of the R″″ type. EP 0 737 660, in contrast, describes the use of quinone methides with substituents of the R″″ type selected from —CN, —COOR, —COR, —OCOR, —CONRR and —PO(OR)2, where R may be hydrogen or an alkyl, cycloalkyl, phenyl or aryl group. In EP 0 737 660 too, the use of these quinone methides with strongly electron-withdrawing substituents in combination with nitroxyl radicals is described.
The use of quinone methides in combination with other known additives for inhibiting polymerization is described by some patent publications. Quinone methides in which R′″=hydrogen and R″″=aryl groups which may optionally also be substituted are described in WO 99/48896 and US 2005/0027150 in combination with hydroxylamines. In contrast, US 2006/0020089 describes these quinone methides in combination with 4-tert-butylcatechol. Quinone methides where R′″=hydrogen and R″″=hydrogen or an alkyl or aryl group in combination with hydroxylamines and catechol derivatives are described by Eldin in US 2004/0034247. Nakajima et al. describe, in addition, sulphonic acids and quinone methides where R′″=hydrogen and R″″=phenyl group which may optionally be substituted for inhibiting polymerization, and it is also additionally possible to use stable free nitroxyl radicals.
A composition of additives for inhibiting polymerization is described by WO 01/40404 A1. This describes a composition consisting of a hydrogen donor or an electron acceptor and a nitroxyl radical, for which conceivable electron acceptors include quinone methides.
Ma et al. describe, in US 2006/0283699, a polymerization inhibitor composition which comprises at least one nitroso compound as a polymerization inhibitor. This polymerization inhibitor composition may comprise, among other substances, nitroxyl radicals and quinone methides.
A polymerization inhibitor composition consisting of at least one C-nitrosoaniline and a quinone imine oxide and at least one compound, selected from compounds including quinone alkides, nitroxyl compounds, is described by Benage et al. in WO 02/33025 A2.